This invention relates to the testing of circuit breakers used in connection with ground fault circuit interrupters. More particularly, the invention allows the testing of multiple ground fault circuit interrupter circuit breakers in a panelboard assembly without opening the panelboard enclosure while using a minimal number of external actuators.
In a three wire system, a ground fault circuit interrupter (GFCI) circuit breaker constantly monitors electricity flowing into a load to sense an imbalance between the line current flowing into the load and the neutral current flowing out of the load. Any difference in these values, beyond a certain threshold, is assumed to be caused by an improper (i.e., unwanted or dangerous) connection between the line and ground wires. The GFCI unit actuates the circuit breaker to remove power from the load when the threshold value is exceeded. A trip lever is included on the circuit breaker, allowing the breaker to be turned on or off, or to indicate that the GFCI has tripped (actuated) the breaker. A manual test switch is included on the GFCI circuit breaker to verify its proper operation.
Two standard threshold current values are used in circuit breakers, 5 milliamps and 30 milliamps. Typically, a circuit breaker which trips for fault current values of 5 milliamps or greater is called a "ground fault circuit interrupter circuit breaker". A circuit breaker which trips for fault current values of 30 milliamps or greater is sometimes called an "Equipment Protection Device". In the interest of brevity, the present disclosure refers to both devices as GFCI circuit breakers.
In current panelboard constructions, the correct operation of a GFCI circuit breaker is verified by depressing the test button on the circuit breaker. This often requires opening the panelboard cover to access the circuit breaker. Alternatively, external test actuators can be added to eliminate the need to open the panelboard cover. Depressing the external test actuator will in turn actuate the test button on the circuit breaker.
GFCI circuit breaker test procedures are further complicated in panelboards used in hazardous Class I (gaseous) and Class II (dust) locations. Testing procedures in these hazardous locations are governed by National Electrical Code (N.E.C.) classifications for areas where the use of electrical equipment can cause ambient gas or dust to explode. In these areas, circuit breakers must be enclosed in an explosion-proof panelboard. The National Electrical Manufacturers Association also provides standard ratings regarding the capability of a panelboard enclosure to withstand potentially hazardous environmental conditions, such as water and corrosion.
The purpose of an explosion-proof panelboard enclosure is to minimize the hazards of electrical arcing in environments where such arcs can cause an explosion. Common plant safety procedures require areas to be classified safe before opening any explosion-proof panelboard enclosure. Consequently, opening the enclosure to perform routine circuit tests on GFCI circuit breakers is not practical.
To avoid the problem of opening the panelboard enclosure, external test actuators can be used to actuate the GFCI test button. However, this design requires that each individual external actuator meet the construction requirements for hazardous location equipment. For each test actuator, a shaft has to penetrate the panelboard enclosure. Each shaft penetration through the enclosure must meet the flamepath length and tolerances of applicable national safety standards. Complying with these standards can be cost prohibitive. Furthermore, each additional shaft reduces the overall reliability of the explosion-proof panelboards by increasing the number of openings in the enclosure that can corrode. In wet areas, each shaft increases the possibility of water leaking into the enclosure.
Two types of shafts may be used to meet flamepath requirements, smooth-sided cylindrical shafts and threaded shafts. Smooth-sided cylindrical shafts are capable of either rotational or transnational motion. Threaded shafts are rotated during operation.
Explosion-proof construction means that each shaft has to prevent flamepath propagation through its entry. For smooth-sided cylindrical shafts, this is accomplished by providing a close tolerance fit between the shaft and its mating bushing and specifying a minimum length of shaft-bushing contact. For threaded shafts, this is accomplished by specifying a minimum number of engaged threads between the shaft and bearing. If the panelboard is to be used outdoors, additional weathersealing preferably would be added. Typically, close tolerance shafts, bearings, and bushings are made of stainless steel to minimize corrosion buildup in the shaft-bushing or shaft-bearing interface. Each shaft assembly requires precision machining, which also adds to the cost of the test circuit.
The present invention substantially reduces the problems and costs associated with testing GFCI circuit breakers installed in panelboards. This is accomplished by minimizing the number of shafts required to test the GFCI circuit breaker. In the present invention, only two shafts are necessary to actuate a selector switch and one push-to-test button. In the alternative, one skilled in the art will recognize that only one shaft penetration is necessary if a combination push button/selector switch device is used. The selector switch eliminates the need for individual push-to-test buttons and shafts for each branch circuit incorporating a GFCI circuit breaker. This feature minimizes machining and parts, thus reducing cost for explosion-proof panelboards. One skilled in the art will immediately realize that this structure is not restricted to explosion-proof designs, but also can be incorporated into ordinary circuit breaker panels.
Other objects and advantages of the present invention will become apparent upon reading the following description.